all right all now that we've discussed air masses it's time to talk about what happens when different air masses Collide and so when that happens we start talking about fronts and frontal boundaries so what is a front when we talk about it well a front is just the term we use to describe when two or more unlike air masses come together remember what I said two or more you have to have at least two but you could have more than that working together in system so where those unlike air masses come together there's going to be a boundary between those different types of air because their temperature and the humidity characteristics are different and therefore you'll have density differences between the air masses so one air mass is more likely to want to go up one air mass is more likely to want to go down based on their density so let's talk about how we find these things on weather maps first when you look at a weather map you can out the front even if it's not labeled most of the time because you can start to see differences in temperature on each side and you can start to see differences in things like dupoint and humidity and then the other thing is the air masses are coming from different Source regions so they're likely to have different wind directions on each side of the boundary for example an air mass coming out of the gulf may have southeasterly winds coming with it an air mass coming from Canada may have North or northwesterly winds associated with it so let's look at this here on this actual weather map and you see a cold front here designated by the blue with the triangles pointing in the direction that it's moving and you have weather stations on each side of this indicating what's going on so the first thing we can look for is Major temperature changes so you can see on the other side of the cold front behind the cold front the temper is only 25° and out ahead of that it's 46° so there's one thing we can look for are the temperature changes we can also look for changes in the moisture content itself and so you can look at the dupoint or the relative hum to see those kinds of things we can also see changes in the wind direction so remember here we can see the winds coming out of the Southwest generally whereas in behind that front right in here we see winds coming from the Northwest we can also have pressure changes so remember our storm systems right along the frontal boundary are associated with low pressure and that low pressure gives rise to uplift clouds and potentially storms depending on how the front is moving and how much moisture is associated with the system in behind our cold fronts will often wind up with high pressure systems that are getting moved from from Canada and then of course with our major frontal systems where there's big pronounced differences on each side of the boundary you will have clouds along the frontal boundary or just ahead of it and you'll wind up with precipitation as a result of uplift remember some of that moisture that's getting lifted up will form clouds and the clouds eventually will drop precipitation if it is heavy enough to overcome the wind currents so let's talk about the different kinds of fronts that we actually have when we look at weather and forecasting okay so our fronts are named and this is pretty basic by the type of air that's replacing the other type of air so a cold front is naturally when cold air is coming in and replacing air that's warmer than it on a map like we just talked about previously it's going to be designated with triangles point in the direction of movement and it's going to be blue if it's a color map so looking at a cold front again colder temperatures in behind and it's replacing warm fronts out ahead so looking at what this looks like relative to an applied case here in Texas there's a cold front on the bottom up by Dallas and it's pushing its way further south Southeast and that cold dense air is plowing through that warm air air remember cold dry air is associated with higher pressure warm wet air is associated with lower pressure that's also related to density so remember the denser colder air mass wants to stay closer to the ground and the warmer wetter air is going to be more buoyant or less dense and it wants to get forced up and along that boundary you're going to get storms as that warm wet air gets forced up and the moisture condenses so our cold fronts are going to be our fastest moving fronts and this is why our cold fronts tend to be associated with more severe weather than warm fronts because they move faster they have a steeper front which means essentially along this boundary that you see here the air is getting forced up more quickly and that's why we're more likely along cold fronts to get severe weather because we're going to get more intense development Chon nimbus cloud development which is more associated with Storms remember when that air gets to the upper levels it can get four out through advection and so this is why sometimes cirrus clouds and longlasting contrails and things like that out ahead of these systems can indicate we're about to get a cold front that is going to roll through so remember because of the intensity the steepness of that front and how quickly and intensely that uplift is occurring our cold fronts are much more likely to result in severe weather and major thunderstorms what happens after cold fronts well most people coming into this class can tell me Well when a cold front comes through it gets colder so we all know that but we need to know other things as well okay one thing remember is that the pressure Falls as that air is getting forced up along the boundary and that's why we said all our storms are low pressure systems that's true not just for cold fronts but also for warm fronts or other fronts that we talk about remember too the wind conditions are going to change so depending on where this cold front is moving through the winds are generally going to have some southerly flow out ahead of it and then in behind it we're going to have some Northerly flow usually because these systems are coming down from Canada remember too that our humidity conditions are going to be very different if this is replacing air in Southeast Texas the air that it's replacing is going to be much more humid and the air in behind a cold front is going to be much much drier because it's coming from Continental areas and replacing areas that are close to Maritime regions so looking at what this looks like in terms of the system right here's our cold pressures or cold front with our high pressure system pushing down in this region right the arrows are pointing in the direction that this is moving in the center of the country where that high pressure is dominating REM remember air is sinking and so the sky is nice and clear along parts of this boundary though depending depending on what air this cold front is colliding with you're getting precipitation along those boundaries and the center of our low pressure system is right in here and we can see during this time of year there's some rain along there but also some snow and ice in behind so once cold fronts pass because they're more associated with high pressure in behind the system and that means air is descending and it's drier air in general those things with sinking air and drier air are not good for cloud development and so sometime after a cold front passes you're likely to experience nice Clear Blue Skies in Behind These systems so can you locate where the cold front is on this map on this given day you should be able to tell roughly just from where the temperature is and if you told me it's somewhere around that whitish boundary between the pinkish and bluish colors on this map there's a line right there that's roughly where the cold front is located I may have not drawn the line perfectly but you get the point the boundar is located somewhere in there on this given day so when we look at cold fronts we can also see what happens when the air actually comes in contact with each other and we get warm moist air that gets forced up remember and cold dry air on the other side with that front coming through so that colder drier air is going to be associated with less clouds because there's less moisture and air is pushing down in behind the boundary itself right along the boundary that warmer wetter air is getting forced up and that is where we are getting our energy for clouds to form and eventually storms so we get our cumulon nimus clouds again our big storm clouds because of that cold front development along those steep fronts with rapid uplift and Rapid movement of moisture into the atmosphere so when we talk about fronts and their development when fronts weaken we talk about using the term frontalis and when fronts strengthen we talk about frontogenesis so you can remember kind of front's building Genesis your origin right these types of things but they can break down over time if the fronts uh weaken because as air becomes more alike or as the air masses move further if they become more like what they're in contact with then you may not have is pronounced a frontal boundary all right we've talked about cold fronts and so now let's talk a little bit about warm fronts notice right away that the cloud types are different here okay yes way out in front of that you're still getting serus clouds based on air getting pushed up but notice that this is not as steep a boundary where warm air is gradually moving into rep Place cold air okay this is going to move more slowly okay and so we're not going to have as steep a front and so we're more likely to have stratus clouds and Nimbo stratus clouds as a result of our warm fronts and the uplift of moisture along these areas and remember warm front will be designated with closed half circles and the circles are pointing again in that general direction of movement so in this case this warm front is moving this way so I told you these move slower okay so the velocity of movement is not as high as cold front and what's happening here in this case is our cold dry air is gradually getting replaced by warm moist low pressure systems and to be replaced there's some kind of High Press system that's helping to steer these warm humid air masses into the continent so in our situation what is most typically happening in Southeast Texas and the Southern United States and Eastern United States is that empty air is getting pushed into the land and is replacing our c air that's come down from Canada so looking at this situation here's some Mt air on the bottom that is pushed up through Tex Arcana and so it's gradually going to push further north and Northeast and we get our steady rain but not as intense a rain or not as intense as storms along that boundary so here we see that this frontal system is affecting Nashville Tennessee so some things to remember about our warm fronts is that there not going to be as severe so you may get a steady amount of rainfall with these but you're not likely to get as severe thunderstorms or anything like that so what happens is before the actual front moves through we're generally going to have movement in our CP air masses coming down from somewhere in the north but because these are coming out of the gulf largely when we talk about them in the US there will be some southernly flow after that frontal system system moves through remember that because this air is coming out of the gulf okay when we talk about most of our warm fronts in the eastern half of the United States the humidity and dup Point are going to go up because there's more heat around to evaporate water and hold it as a vapor or in gas form and remember that when you add more water to really warm air you can get higher humidity and dupoint levels as a result so looking at a cross-section of what a warm front looks like again remember these can also move through during winter and that moisture colliding with that cold air can cause different forms of precipitation depending on where you are in the boundary itself so we talked about the different forms of precipitation earlier in class but this is a situation where you could have different forms of Frozen precipitation you could have sleep where the atmosphere is a little colder freezing rain out a little ahead of that and then you could also have snow in behind right or out ahead of that boundary where it's much colder so looking at what's happening here here's our warm front moving through our cold air on the other side and you'll notice where the air is cold we have freezing temperatures and so you can see that there's some snow occurring in these areas okay out ahead while there's other types of precipitation forming in closer to the boundary itself our snow our sleep and our freezing rain okay so there's that cross-section right here of how this works as warm fronts move through during the winter months now we have cold fronts and warm fronts but something that happens here in the US is looking at uded fronts I told you we have to have at least two air masses but often times in the US we get uded fronts that form where there's a cold front that moves through and then a warm front and then a cold front in behind so it moves through in kind of these cycles of three where you have a cold warm and cool air mass so what happens is we get occlusion in the middle because the cold fronts are moving faster and so it kind of squeezes that warm air mass in the middle and causes uplift on each side rooted front is designated with triangles and half circles to designate that there's multiple air masses moving through so there's cold type occlusion okay so this is where the coldest air mass is actually in behind Okay when we look at what's occurring in the system and so the temperatures go from cool to warm to very cold when the last air mass comes through is the coldest one so what happens here is you get higher pressure associated with the cold air mass in front low as the frontal systems move through as that warm air mass gets forced up in between and then it switches back to high pressure once that coldest front has moved through and so what ultimately happens remember is no different than what we have with cold fronts if our coldest air mass is in behind then we get less humidity and less moisture once this system is moved through so looking at a cold type of cluded front I'm just showing you here I want to emphasize that this air mess in behind is the coldest one and so that's why we talk about it as a cold type the coldest air masses last this one out ahead is cool but it's not as cold as the one that is going to follow in behind at the end so it's important to know that we have major rainfall along those boundaries where the warm air in the middle is getting forced up and remember where there's a more pronounced boundary like right in here the difference between 28 degree air and 70° air is pretty drastic so you could be getting some pretty intense rainfall and storms along that boundary or again if you're you're in behind that air mass and it's 28° you could have some pretty intense snowfall our warm uded type just means that the colder air mass is in behind and the coldest is out in front but remember the warm air in the middle is still not moving as fast as the air that is moving along with that cold front and so it still gets squeezed up in the middle so when we look at a cross-section here of what's happening with a warm type uded system remember out on the front edge we're going to have a little bit colder air than the air in behind this time okay so we're still going to have a warm front associated with this because there's warm air right in the middle so right along this boundary here and then we're going to have a cold front in behind and you see that reflected also in the image above and looking at the cross-section so looking at it again in more of a weather map view and looking at what the temperatures look like remember now the coldest air is out in front okay and so you could have another you could have another cold front that's drawn out here associated with that air out front I know it's red but you get the point in the middle is the warmest air and now in behind is the cooler air in this case so a warm type again we have that cool air in the back and the coldest air out front another type of front that you can get is something called the stationary front and this will affect us and cause major rainfall along the Gulf Coast sometimes and you see here that this system is set up over Texas and Louisiana on this given day and what happens with the stationary front is there's just not enough movement from either one of the air masses to actually push the other one out of the way but there's still different air on each side of these boundaries so on this side you're going to have colder drier air on this side you're going to have warmer wetter air so you still get a boundary along those air masses where there's precipitation and cloud cover right along the difference between those two systems and if this doesn't move for a while and that's why we talk about it being stationary then you can get some thunderstorms that keep happening over and over again affecting the same areas and you can get localized and Regional flooding as a result okay so just like you see on this weather map on the right stationary fronts are going to be indicated with red circles on one side and blue triangles on the other indicating there's cold air on one side and warm air on the other but when we list them like this it tells us that neither one of those is dominating or moving the other out of the way another thing we need to talk about in Texas is something called the dry line if I was teaching this in Wisconsin I might not talk about it as extensively until we talked about tornadoes or things like that but in Texas we have to be aware that the CT air mass and that Mo moisture coming out of the gulf will collide with the dry air coming out of the Southwest so you see in here the dry line okay and they may have drawn this to extend a little far north it's likely a little bit further south often times but the dry line is designated to show us that warm dry air that's moving out of the southwestern part of the us and it's going to come into contact with more moist air now remember even though both of these air masses are very warm air coming out of the Sonoran Desert and air coming out of the golf of Mexico remember there's going to be density differences related to the humidity and remember air with a lot of moisture in it when that waterers a vapor it's actually less dense than other air so when this CTS air slams into MTG air you're likely to get major uplift of that Gulf Air and there's just a lot of total energy around because both air masses are warm so not coincidentally we hit major thunderstorms and tornado es associated with where this dry line can push in and come into contact with that warm humid air so the dry line extends in the Southern Plains and right down through Central Texas right into here where that dry air coming out of the Southwest hits that warm humid air that comes in from the Gulf of Mexico and if the conditions are right and you can tilt that air more vertically in terms of rotation you can get tornadoes that again is something we'll talk about more a little bit later so you can see this really pronounced on this given day where we're looking at the dupoint temperatures so you'll see these orangish colors down here in the Gulf all in here all these orange colors near the coast we have dup points that are in the 70s so it's going to be very uncomfortable and very humid and meanwhile look what the D points are out here in West Texas and New Mexico if you look at the scale over here on the right hand side we're talking about dup points down under 25 degrees so you're talking about air with a 25 degree dup point right slamming into air with a 70° D point and you are likely to drive some major thunderstorm development and uplift when you have air that's that different in humidity because there's going to be major density differences and so if you were to look at the radar what kind of things do you think you would see right along that boundary if you told me a well pronounced frontal boundary with thunderstorms if you told me a well pronounced boundary with thunderstorms then you're right there it is the dry line developing over Central Texas and that line of thunderstorms that is then going to push across and get moved largely by Westerly flow as it moves out over Louisiana and usually will dissipate a bit because the dry air will start to gain more humidity and so sometimes this will start to die down a little bit as it approaches the east coast and then gets out toward The Atlantics so that's frontal systems that's going to set the stage for how our major severe weather storms move across much of the United States but we'll also talk about hurricanes and things that are unique to Texas and the Gulf Coast as well